Major problems have been encountered in the past in maintaining locking engagement of the bolt and nut of a fastener assembly after the bolt and nut are tightened. In order to solve these problems, a washer is generally used between the bolt and the nut. For example, pages 45 to 46 of “Story of Screw” (author: Akira Yamamoto, Published by Zaidanhojin Nihonkikakukyokai, on Oct. 30, 1990) disclose a technique using a plane washer or spring washer. Laid-open Japanese patent publication Tokukaihei1-182614 discloses a special form of washer adapted to a spherical surface.
In the above-identified tightening structures, the resulting locking effect after tightening the bolt and the nut, is insufficient, and unlocking between the bolt and nut occurs inevitably. Therefore, this invention has been created in order to solve this problem and provide a tightening structure which can perform the function of locking more firmly, as well as strengthen the resulting joint after tightening of the bolt and nut.